The present invention relates to a concept of providing a combination means of feeding, heating and drying virgin aggregates and, at the option of the operator, having the heated virgin aggregate directed to a batch plant tower for batch mixing, or in the alternative, to a continuous drum mixer where recycled asphaltic pavement, liquid asphalt cement, fillers and other additives may be blended in the preparation of asphalt paving materials. In the past, asphalt plants have generally been of two types, the Batch type, and more recently the Drum Mix type. Batch type plants typically have hopper feeders for feeding different sizes of unweighed cold aggregate materials by means of a cold feed conveyor into the dryer. The dryer typically involves a large elongated drum having a burner directed into one end, with the aggregate being fed therethrough from the other end, and providing rotation of the drum with flights for lifting the aggregate material and dropping it through the hot gases and causing it to be dried. As the material flows through the drum, toward the burner, the hot air and gases flow in the opposite direction to the flow of materials (counter flow) exiting out of the other end of the dryer into a ducting system which is directed towards a wet wash, mechanical collector, or fabric filter dust collector for cleaning the air and gases prior to their discharge from a stack. Meanwhile, the dried aggregate materials are fed up a bucket type hot elevator to a batching tower, where the material is passed through screens for separating it into sizes and storing it into separate compartments whence it is withdrawn pursuant to various formulas into the weigh box. After weighing, the aggregates are dropped into a mixer at the bottom of the tower where the exact amount of hot liquid asphalt is added, along with mineral fillers, and limited amounts of recycled asphaltic materials and mixed to specified time durations, prior to loading into a truck. This type of plant has been the most commonly used around the world, but has the disadvantage of being able to process only limited amounts of recycled asphaltic pavement materials, with substantial environmental problems, and at a higher production cost than a Drum Mixer.
The other concept of asphalt production, The Drum Mix Plant, has a feed system which preproportions the ratios, or actually weighs in motion, the various aggregates onto the cold feed conveyors, feeds it into a rotating elongated drum dryer, where a burner is directed into the same end as the cold aggregates, (Parallel Flow). As the aggregates tumble and progress through the rotating drum dryer, progressively away from the burner, they absorb heat and shed moisture. As aggregates progress into the drum, at some point therein liquid asphalt is injected which mixes with the aggregates through the tumbling action and exit the dryer discharge as a finished mixture ready to be loaded into trucks for hauling to the paving site. This type of drum mix plant is most commonly used to process larger ratios of recycled asphaltic pavement (RAP) which is introduced into the elongated drum dryer at some point prior to injection of the liquid asphalt materials. The disadvantage of this drum mix plant is that both the liquid asphalt, as well as the recycled asphaltic pavement, are introduced into the same single elongated drum dryer into which the intense fire is released from the burner, and subjected to the high velocity, and elevated temperatures of the plant exhaust system. The high temperature from the flame and combustion gases cause burning of the liquid asphalt, degradation of quality of the finished product, release of volatiles from the liquid asphalt, as well as the recycled asphaltic pavement materials, generation of blue smoke and other severe pollutants, release of odors, and also can cause fires and explosions from the gasified volatiles which tend to recondense into liquid form in the fabric filters of the exhaust system.
Prior U.S. Pat. No. 4,262,429 deals with a method of and apparatus for drying materials which uses coaxial cylinders with one being fed moist fine aggregates and the other being fed moist coarse aggregates for mixing and drying the aggregates. It does not deal with the manufacture of paving asphalt nor the use of recycled asphalt and does not provide the pollution reduction (for asphalt plants) accomplished with an exhaust feedback loop and dust collection chamber.
The present invention is directed to an asphalt plant which can process a mixture of recycled asphalt and virgin materials or can process only virgin materials without oxidizing, distilling or burning the liquid asphalt, or that of the recycled asphaltic paving material, and without releasing volatile matter, particulate pollutants, or odors. It utilizes the energy-efficient Counterflow process to dry the materials in the drying section of the rotating drum. After heated and dried materials pass the position of the burner they enter into the mixing, rotating drum where liquid asphalt, as well as recycled asphaltic materials and mineral fillers are introduced to blend with the heated virgin materials and mix in a static environment. Thus, the asphaltic components of the mix do not sustain degradation due to the temperatures of the burner gases, nor does it allow for contamination of the exhaust system with gasified or liquid constituents of asphalt, asphaltic odors, recondensing petroleum vapors, or particulate emmissions from the mineral fillers. Any vapors generated in the static environment of mixer section are withdrawn and fed into the adjacent burner where they are incinerated while enriching the fuel of the burner.
It is accordingly an aim of the present invention to provide an aggregate and virgin asphalt plant which can utilize recycled asphalt materials and produce a superior product utilizing substantial amounts of normally wasted energy, and dramatically reduce release of environmental pollutants, as well as reduce fire and explosion hazards.
In U.S. Pat. No. 4,600,379 for a drum heating and mixing apparatus and method, a combination direct and indirect fired drum heating and mixing apparatus is provided for the same purposes as the present invention. The present invention is an improvement over this heating and mixing apparatus which has a concentric outer drum mounted around an inner drum and has an aggregate input from the material output of the inner drum and an aggregate output therefrom at the other end of the outer drum so that aggregate material can pass through the inner drum and then pass between the inner and outer drums. A burner mounted at one end of the inner cylindrical drums directs a flame thereinto for drying an aggregate. An exhaust gas feedback siphons vapors and gases emitted in the space between the two drums back into the flame for incineration through the system burner. An asphalt cement injection nozzle injects asphalt cement into the aggregates passing through the concentric outer drum while a second aggregate feed can direct recycled asphalt pavement material into the concentric outer drum. In contrast to this drum heating and mixing system, the present invention directs the aggregates through the equivalent of a prior center drum and into a second drum mounted concentric but behind the center drum for receiving a direct pass through the aggregates. The second concentric drum can be separately attached to the first drum or rotated individually.
In addition to this patent, the prior U.S. Pat. to Rohrbach, No. 4,342,554, shows the production of expanded clay and shale which uses a larger drum placed behind a smaller angled rotating drum. The Maeda, et al, Pat. No. 4,462,793, is for a calcining lime system and the Drugge Pat. No. 4,168,951 is for a cintering apparatus, while the Cnare Pat. No. 4,367,065 is for drawing coal. Each of these patents show burners used for different purposes but having rotating drums angled toward the flame which feed the material being dried in one end toward the flame. The Labriot, et al, Pat. No. 4,286,944, feeds material past a center burner in one of its embodiments. The Deckebach Pat. No. 4,439,141, shows another related apparatus. The present invention is an improvement over my prior drum heating and mixing apparatus and method as taught in U.S. Pat. No. 4,600,379 and provides for the easy addition of a second drum positioned outside of the flame to receive the dry aggregate from the drum having the burner therein so that the mixing takes place outside of the flame.